I quite agree about that. But not sure about the rest of your post. Sure in part I am sure what you say is a factor, but I think it's also about encouraging a regular replacement cycle. The fact is when your device is looking old, they want you to replace it with another one, so there's no incentive at all to make it upgradeable, whether or not there's a market for such.
Right, but it's not a cycle driven by Apple - they merely look at what the market does.
Even in the world of the highly-upgradable PC, only a very small minority ever upgrade their computer rather than simply replacing it, even when they have the ability to do so.
Upgrading a computer is simply not something the bulk of people do. This is true when computers are upgradeable (like regular PCs) and is true for Apple's machines - Apple simply takes advantage of this to enhance the design of their machines.